Thermionic converter



-; 1969 w. E. HARBAUGH THERMIONIC CONVERTER Filed March 29, 1966 TEMP.PRES. CURVE FOR SINGLE RES AT TEMP; TE

PRES; TEMF? CURVE RES. AE AT E'M. TEMP.

le'po 2000 T TEI E2 TEMPERATURE,K

m T E3 wmnmmmmm INVENTOR.

WILLIS E. HARBAUGH BY F/g.2

United States Patent 3 Claims ABSTRACT OF THE DISCLOSURE Thermionicgenerator having a radioisotope heated emitter, a controlled temperaturecollector adjacent thereto, and equal area adsorption means that arespecifically located near the collector and emitter at differenttemperatures for the interchange of alkali metal vapor therebetween asthe emitter temperature decreases due to the radionuclide decay of theradioisotope heat source.

This invention relates to thermionic energy converters and moreparticularly to thermionic energy converters containing a source of gasor metal vapor. The invention described herein was made in the courseof, or under a contract with the US. Atomic Energy Commission.

In the field of electrical power generation thermionic converters are animportant and growing electrical energy source. These converterscomprise an electron tube having a cathode and an anode for convertingheat energy to the cathodeto electrical energy in the form of a voltageproduced by the tube itself between its emitter cathode and collectoranode terminals. The materials of the cathode and anode are usuallychosen to provide a cathode having an electron work functionsubstantially higher than that of the anode thereby to produce aninternal electric field for accelerating electrons from the cathode tothe anode. In order to facilitate travel of electrons from the cathodeto the anode, the space charge of the electrons between the cathode andanode may be neutralized by the use of positive ions. One source of suchions may be an alkali metal vapor having a low ionization potential,such as cesium, introduced into the interelectrode space.

Various proposals have been made or used to provide the required vapor,including the systems in which the alkali metal vapor has beenintroduced from an envelope containing a liquid pool of alkali metal butthis has required an appendage communicating with the interelectrodespace of the device. Also, heating of the alkali metal to a propitioustemperature, and/or precise external control has been required tooptimize the cesium vapor pressure for various emitter temperatures. Ithas addition-ally been advantageous to provide automatic selftrackinginternal means for optimizing the cesium vapor pressure for variousemitter temperatures.

Accordingly, it is an object of this invention to provide an improvedpool-less thermionic converter and method of operating the samecharacterized by means providing automatic tracking of the cesiumpressure within the converter.

It is another object to provide a pool-less thermionic generator havingmore than one adsorption area for the interchange of interelectrodealkali metal vapor therebetween wherein the rate of change of thepressure of the vapor with respect to the emitter temperature is afunction of the adsorption areas.

It is another object to provide in a pool-less thermionic generator twometal vapor adsorption areas that provide the vapor pressure at anoptimum value for a wide range of emitter temperatures without the needfor external con- 3,426,221 Patented Feb. 4, 1969 'ice trol so as toprovide a simple, self-tracking, effective and practical thermionicconverter system.

It is another object of this invention to provide a poolless thermionicconverter having two large high tempera ture alkali metal adsorptionareas that act together to establish thermionic converter pressureaccording to the ratio of their areas and the coverage of alkali metalon their surfaces.

This invention provides a method and apparatus for the practical andefiicient production of electrical power from heat derived from theradionuclide decay of radioisotopes for a long operating lifetime,remote power load for space applications. The method and constructioninvolved in this invention utilize standard and well known techniquesand apparatus and is highly flexible for a broad range of applications,alkali metals, electrodes, emitter temperatures, vapor pressures, andheat sources. More particularly, this invention involves the use of twopool-less, large, high temperature, alkali metal adsorption means thatare specifically located near a collector and emitter at ditferenttemperatures and have large equal area-s that interchange alkali metalvapor therebetween as the emitter temperature changes. The adsorptionareas are arranged in a thermionic converter in one embodiment, to besaturated with alkali metal vapor in the ratio of about one half gram inthe adsorption areas to about 10 micrograms of alkali metal vaporoutside the adsorption areas. With the proper selection of adsorptionareas and maintenance of less than a monolayer of alkali metal vapor onsurfaces outside the adsorption areas, the desired pool-less simple,effective and efiicient self-tracking electrical power generation isachieved over a wide emitter temperature range.

The above and further novel features of this invention will appear morefully from the following detailed description when the same is read inconnection with the accompanying drawings. It is expressly understood,however, that the drawings are not intended as a definition of theinvention but are for the purpose of illustration only.

In the drawings where like parts are referenced alike:

FIG. 1 is partial cross-section of the thermionic converter of thisinvention;

FIG. 2 is a graphic representation of pressure in torr vs. temperaturein K. of the system for self-control of the cesium pressure in theapparatus of FIG. 1;

FIG. 3 is a schematic illustration of the processing apparatus for theconverter of FIG. 1.

During the development of this invention a thermionic generator wastested having no metal vapor reservoir, but this device was unsuccessfulsince vapor material was lost due to the extreme reactivity of the vaporwith the internal generator components or other causes and theelectrical output was critically dependent on a constant vapor pressure.Thus this generator was not suitable for use with a decaying radioactivesource. Other tests showed problems existing with internal reservoirs.It was discovered in accordance with this invention, however, thatparticular communicating adjacent internal adsorbers, at the collectorand emitter temperatures, self-track thus to be particularly suited fora decaying heat source.

This invention is particularly adapted for the production of electricalenergy from the heat resulting from the radionuclide decay of aradioisotope. The converter and system of this invention, however, mayalso use other heat sources. The converter of this invention is suitablefor use in space for energizing radios or other scientific payloads butit also may be used in any location where a dependable, long life orremote electrical power source is desired.

In understanding this invention reference is made to FIG. 1, which showsa thermionic energy diode converter 11 having an anode emitter 13,cathode collector 15 and interelectrode alkali metal vapor to provideadequate space charge neutralization. This is a cylindrical thermionicconverter having nickel as the collector material, molybdenum as theemitter material and synthetic sapphire as the material for insulator 17between the two electrodes 13 and 15. An envelope 19, comprising portion21 made from NiFeCo alloy, such as Kovar brand alloy, and nickel portion23 prevents any loss of the alkali metal vapor, since any loss of thisvapor material to the atmosphere will cause the power output of theconverter to drop.

Flexible metal diaphragm provides differential expansion and contractionbetween the converter parts to maintain a constant interelectrode widthin gap 27. Ceramic insulators 17 and 31 separate the working centralportion 33 of the converter 11 from the top portion 35 and bottomportion 37 thereof. Emitter contact ring 39, having bolt holes 41therein provide support for the whole converter structure and coolingjacket 43 thereof, which has means M for circulating a suitable coolingfluid therein to maintain a constant collector temperature. One means Mis the heat pipe disclosed in U.S. Patent 3,229,759, but variablepumping means, thermostatically controlled shutter means or variableheating or cooling means may also be used. Pinch off tube 45 providesmeans for introducing the interelectrode vapor. The interelectrode vapormaterial enters the converter 11 through tube 45 and the tube is thenclosed.

A suitable heat source, such as a radioisotope sources, placed insidethe cylindrical chamber 51 of emitter 13 provides heat energy to driveelectrons from the emitter 13 to the collector 15 so as to produce anelectrical power producing electrical differential between these twoelectrodes, while remaining to provide adequate space chargeneutralization.

This invention provides two internal adsorption means 61 and 63 for thevapor to provide automatic tracking of the vapor pressure within theconverter 11. To this end these adsorbers are large in area,advantageously equal in area, adsorber 61 is adjacent to the emitter 13at the temperature of the emitter 1'3, adsorber 63 is adjacent to thecollector 15 at the temperature of the collector 15 and there is aninterchange of vapor material between the adsorbers 61 and 63 as theemitter temperature de creases due to the radionuclide decay of sourceS, whereby the adsorbers provide automatic optimum self-tracking of thevapor pressure as the emitter temperature decreases. The system of thisinvention changes the Cs pressure automatically for a constant emittertemperature and for new emitter temperatures by automatically trackingto the new required Cesium pressures. This system works best with aconstant collector temperature but normally with a radioactive heatsource the emitter temperature changes only slightly with even lesschange in the collector. However, this invention will automaticallytrack much better than the systems known heretofiore even if both thecollector and emitter temperature change.

In understanding 'how these adsorbers 61 and 63 provide the desiredself-tracking, reference is made to FIG. 2. Curve PP illustrates thatthermionic converters require a particular pressure for every emittertemperature for optimum performance. An adsorber at emitter temperaturedoes not provide the proper interelectrode pressure for optimumconverter operation since it would follow the pressure temperaturerelationship B-B. By utilizing a first adsorption means 61 adjacent theemitter 13 and a second large area adsorption means 63 located at thecollector 15 which is essentially at constant temperature the twoadsorbers 61 and 63 work together to establish converter pressureaccording to the ratio of their areas and the coverage of the vapor ontheir surfaces.

The relationship that determines the pressure is A 9 +A =K=c0nstantwhere A is the area of the emitter adsorber 61, A in the area of thecollection adsorber 63, G is the vapor coverage of the emitter ad- 4sorber 61 and 6 is the vapor coverage of the adsorber 63.

The fact that the total amount of vapor material in the system must beconstant means that any change in emitter temperature will require aninterchange of cesium between the adsorbers 61 and 63 since the emitteradsorber 61 cannot now follow a constant coverage curve. The modifyingeffect of the collector adsorber 63 can be made greater or smaller,depending on the size of its area compared to that of the emittertemperature adsorber 61. The proper ratio of adsorber areas to make theresultant converter pressure equal to the desired pressure curve P-P canbe solved graphically from the constant coverage curves of FIGURE 2.Actual tests have shown that the desired ratio is very close to unityfor optimum converter operation. Moreover, during operation theseadsorbers 61 and 63 provide automatic tracking of the vapor pressure atthis optimum value for a wide range of emitter temperatures, such as areprovided by the decay of the radioactive source in emitter 13, withoutthe need for external control. In one embodiment, the cathode anodespacing is 0.025 cm.

Cesium may be lost by chemical reactions with residual gases orconverter materials and it can be lost by physical means such ascapillary attraction and condensation. Thus the recited materials andsintered tungsten particles forming adsorbers 61 and 63 are employed.Advantageously the adsorbers 61 and 63 comprise metal sponges havingpore walls formed by sintering the tungsten with other materials such asaluminum oxide particles, both particles being 10- cm. in diameter andhaving in each adsorber a volume of 1 cm. 3 and an adsorbing area of10,000 cm. so that more times vapor material atoms than elsewhere in thedetector can adsorb in the adsorbers. In one embodiment having an insidevolume of 2 cm. outside the adsorbers, there is one-half gram of Cs ineach adsorber and 10 micrograms of Cs outside the adsorbers. For normalconverter life, the pore walls should have a large area for adsorbing atleast ten times as many cesium atoms as are present elsewhere in theconverter. In the rest of the converter, the inside metal surfaces ofthe device are covered with less than a monolayer of cesium so that noliquid or pool exists in the device and this coverage of the internalsurfaces of the device is an equilibrium state between the arrival andevaporation rate of the alkali metal vapor at these surfaces.

The greater affinity of the vapor atoms for surfaces other than thevapor material in its liquid form makes it feasible to store the vapormaterial in an adsorbed state within the device at a relatively widetemperature range, extending from a temperature slightly higher than thenormal condensation temperature of the vapor material to anysubstantially higher temperature at which the converter is in operation.Thus, the temperature at which the vapor material pressure is optimumfor best converter efiiciency may be used even though substantiallyhigher or lower than one or more elements of the converter.

Advantageously, the binding energy of the vapor material atoms to thesponge pore walls is 2.00 electron volts at 553 K. whereas the bindingenergy of the vapor material to a liquid pool thereof is only 0.75electron volt at that temperature.

In establishing the proper cesium pressure initially, specificprocessing techniques were developed. To this end the converter 11 isadvantageously processed in an ultrahigh vacuum system of at least 10"torr to reduce residual gases and to achieve a high degree of vapormaterial purity. With background gas at a minimum the desired preciseamount of material for the vapor enters converter 11 thr ugh tube 45 anda suitable machine pinches tube 45 to seal the vapor material insideconverter 11. To this end, the converter envelope is maintained at ahigh temperature and a hot pinch-off is made to maintain the vacuum atleast as low as 1 torr.

The converter of this invention advantageously comprises a smallenvelope diameter and sapphire emittercollector insulation. Electronbeam welds and tungsten inert gas welds are used only. Where brazes wereused, these were iron-palladium and nickel-palladium brazes without goldor other conventional brazing alloys.

Advantageously, the assembly system has two pumping systems 101 and 102and suitable means 103 for providing heater bombardment power andenvelope heating power. Also, the converter is connected to distillationapparatus 105 for the vapor material through the nickel pinch-off tubeby a bakeable flange 107 therefor. The vapor material als connects to aliquid-nitrogen cold trap 109 and then to an exhaust manifold 111. Asecond liquid-nitrogen cold trap 113 provides a safety device to trapradioactive vapor material before it gets to the pump 102. The pumpingsystems are of the getter-ion type with a pumping speed of 15 liters persecond. The second independent pumping system exhausts the heaterbombardment chamber.-117.

Processing follows the steps, comprising bakeout of the entire exhaustsystems at 500 F., degassing of the converter elements at a low pressureof l-' torr, admission andcontinuous distillation of cesium into theconverter, maintenance of the converter interelectrode material in itsvapor state i.e. in its Ball-of-Fire" m de raising of the emittertemperature to between 1300 C. and 1350 C. and pinch-off of theconverter from the exhaust system while operating and providing aminimum envelope temperature of 150 C. higher than the optimum vapormaterial temperature. Advantageously pinch off is with the emitter at1200" C. and the envelope at 450 C.

In the operation of the converter of this invention, the radioactiveheat source heats the emitter to 1350 C. while the cooling jacketmaintains the envelope at a temperature below that and above 280 C.,e.g. 450 C. Advantageously cesium is the interelectrode material havinga critical condensation temperature of 270 C. One advantageous initialtemperature at the collector is 600 C. and 1350 C. at the emitter. Thepressure at this temperature is 1 torr. However, depending on theemitter temperature which determines the propitious operating pressure,the Cs pressure can be anywhere between .1 torr and about 5 torr. Thelatter pressures involve emitter temperatures from 1260 C. to 1800 C.This produces electron flow from the emitter to the collector and avoltage across the converter output leads.

It has been found in accordance with this invention that heavyundesirable, liquid pools of alkali material are prevented. In thisregard less than a mono-layer of the vapor material and suitable hightemperature adsorbers are used. Also, the lack of welds and materialsthat contain residual oxides have prevented the retention of vapormaterial therein. Moreover, the molybdenum and nickel electrodes andsupports and the main shell have been free from attack by the cesiumwhile the metalized seals and sapphire insulators have preventedundesirable attacks from the vapor material. Additionally, the carefuldegassing and sealing of the converter at the required vapor pressurehave enabled the converter to prevent undesirable losses of the vapormaterial and initial pressure thereof for long periods of time so as toprovide a substantially constant'power output over the wide temperaturerange produced by a decaying radioisotope heat source.

The converter of this invention has the advantage of being useful with awide range of heat sources such as radioisotopes, nuclear fission, solarradiation and fossil fuel where heat output has often varied, as well assystems having sump controls and the like for providing a constant heatinput. The system of this invention is particularly advantageous forboth large and small, and low and high power output systems and to thisend it is operable to produce a substantially level power output over awide range of heat inputs. Also, it contains vapor material whosepressure varies optimally with temperature while this temperaturemaintains all the inter-electrode material vaporized. Additionally, theconverter of this invention has a high degree of compatibility with aWide range of highly reactive interelectrode vapor materials andprovides little or no loss of vapor mate-rial so as to maintain asubstantially optimum interelectrode vapor pressure over long periods oftime.

What is claimed is:

1. The method of operating a thermionic gas tube having a collector, anemitter and an interelectrode vapor, comprising: baking said tube at 500F., degassing said converter elements at 10 torr, admitting continuouslydistilled cesium into said converter while maintaining said cesium inits vapor state in said converter, closing said converter to maintain aconstant cesium vapor pressure in said converter, and maintaining adifferential temperature between said emitter and collector above thecondensation temperature of said cesium while adsorbing a large portionof said cesium adjacent said emitter and collector substantially at therespective temperature thereof to produce a. substantially constantelectrical power output from said converter between said emitter andcollector over a wide emitter temperature range.

2. A thermionic energy converter for producing electrical power from aheat source, comprising a cylindrical inner molybdenum emitter, aclosely spaced cylindrical outer nickel collector, a pure interelectrodecesium alkali metal vapor, a sapphire insulator between said emitter andcollector, respective porous first and second metal sponges adjacentsaid emitter and said collector formed from sintered tungsten particlesfor adsorbing a major portion f said alkali metal vapor material, anickel envelope enclosing said elements having means for maintaining aconstant collector temperature, and a radioactive heat source insidesaid emitter for vaporizing all said alkali metal vapor and heating saidemitter and collector to differential temperatures above the normalcondensation temperature of said alkali metal material to produceelectron flow between said emitter and collector for producing asubstantially constant voltage therebetween over a wide emittertemperature range.

3. The invention of claim 2 in which the relationship A 0 +A 6 =K=aconstant where A is the area of the emitter adsorber, A is the area ofthe collector adsorber, 6 is the vap r coverage of the emitter and 0 isthe vapor coverage of the collector, A and A being equal and larger inarea than said collector and emitter respectively.

References Cited UNITED STATES PATENTS 2,510,397 6/1950 Hansell 31042,980,819 4/1961 Feaster 313212 3,300,661 1/1967 Talaat 310-4 MILTON O.HIRSHFIELD, Primary Examiner.

D. F. DUGGAN, Assistant Examiner.

